Liquid crystal display elements now widespread chiefly employ a TN (twisted nematic) type display mode utilizing nematic liquid crystals. Under the state of the art, although a TN type display system offers many merits and advantages, it has considerably lower rates of response as compared with light-emission type display systems, such as a cathode ray tube (CRT). Studies on liquid crystal display systems other than the TN type mode have been made, but satisfactory improvements on response characteristics have not been reached.
In recent years, it has been turned out that liquid crystal devices utilizing ferroelectric smectic liquid crystals make it possible to obtain rates of response 100 to 1,000 times higher than those conventionally obtained and to obtain memory of display even after cut-off of an electric source because of bistability (memory effects). This means a great possibility of application to light shutters, printer heads, panel type television sets, etc., and extensive investigations are now being made to establish practical application in various fields.
The liquid crystal phase of ferroelectric liquid crystals belongs to a tilt type chiral smectic phase. Preferred of chiral smectic phases in view of practical use is a phase called chiral smectic C phase (hereinafter referred to SC* phase) having the lowest viscosity.
Liquid crystal compounds exhibiting an SC* phase (hereinafter referred to as SC* compounds) have hitherto been studied, and a number of compounds of this type have been synthesized. However, none of these SC* compounds, when used alone, does not satisfy all of the characteristics required for use as a light switching element for ferroelectric liquid crystal display. That is, the ferroelectric liquid crystal compound is required to have (1) ferroelectricity in a broad temperature range including room temperature, (2) an appropriate phase series in a high temperature region, (3) a long helical pitch, particularly in a chiral nematic phase (hereinafter referred to N* phase), (4) an appropriate tilt angle, (5) low viscosity, and (6) a fairly large spontaneous polarization value (hereinafter referred to as PS value), and to exhibit (7) satisfactory orientation owing to the characteristics (2) and (3), and (8) high rate of response owing to the characteristics (5) and (6).
Hence, under the present circumstances, SC* liquid crystal compositions are used for studies and the like.
SC* liquid crystal compositions are generally prepared by doping a liquid crystal compound or composition (hereinafter referred to as a mother liquid crystal) which shows no ferroelectric properties and exhibits non-chiral smectic C phase (hereinafter referred to as SC) with one or more kinds of chiral compounds (hereinafter referred to as a chiral dopant). According to this technique, however, unless the chiral dopant to be added exhibits considerably large spontaneous polarization, the resulting SC* liquid crystal composition shows too small spontaneous polarization to have a high response rate.
The chiral dopant is not always required to show a liquid crystal phase. However, in order not to make the liquid crystal temperature range of the composition narrow, it is desirable that the chiral dopant exhibits a liquid crystal phase, preferably a chiral smectic phase (hereinafter referred to as SX), and particularly, if possible, an SC* phase, up to a high temperature region. Further, many of liquid crystal compounds capable of exhibiting large spontaneous polarization give a strong twisting force to the liquid crystal phase, thus reducing the helical pitch in phases where a helix appears, i.e., SC* and N* phases, which adversely influences orientation of the liquid crystal compounds. Therefore, when such a liquid crystal compound is used as a chiral dopant, it has been necessary to limit the amount to be added or to control the helical pitch of the SC* liquid crystal composition by addition of a chiral compound which shows twisting in the opposite direction. However, control of the helical pitch has encountered with troublesome problems. For example, if the chiral compound shows twisting in the opposite direction, the direction of spontaneous polarization would also be opposite to offset the spontaneous polarization of the chiral dopant.
For these reasons, it has been demanded to develop a chiral liquid crystal compound which shows spontaneous polarization as large as possible, has a large helical pitch, and exhibits an SC* phase by itself.